Three major studies of childhood obesity are proposed. Study 1, Part A will compare the energy expenditure of free living obese and non-obese adolescents matched for age, sex, and lean body mass (LBM) using the novel technique of 2H218O to measure CO2 production over a 14 day period. If RQ is known, O2 consumption can be calculated. These results will reveal whether mean daily energy expenditure among obese adolescents is reduced compared to non-obese controls. Study 1 Part B will compare the thermic and hormonal effects of overfeeding 1500 kcal of predominately carbohydrate calories for 14 days in obese and non-obese adolescents matched for age, sex and LBM. Daily energy expenditure will be measured by 2H218O. The thermic effects of a meal will be measured by open circuit indirect calorimetry. These studies will determine whether the obese are more thermogenically efficient with respect to overfeeding and should clarify the hormonal mechanisms that lead to differences in energy expenditure. Study 2 will compare the effects of a hypocaloric diet containing (1.5 mg protein + 1.0 gm glucose)/kg IBW/day with a diet of 2.5 gm protein/kg/day in 14 obese male adolescents consuming each diet in a crossover fashion for two 21 day periods. Rates of glucose production will be measured with 6,6-d2-glucose. Net protein catabolism will be measured by nitrogen balance and primed constant infusions of Alpha-15N-lysine and 1-13C-leucine. Comparisons of 3-13C-alanine incorporation into glucose will be used to measure the relative contribution of protein catabolism to gluconeogenesis. Fat utilization will be assessed by measuring lipolysis with d-2-palmitate and d-5-glycerol, indirect calorimetry, and changes in body composition. Our results will yield new insights into the mechanisms by which these diets exert their metabolic effects. Study 3 will develop measures applicable to field studies of pediatric obesity. Part A will examine the relationship of a variety of anth opometric measurements to body fat and fat distribution by measuring body composition with H218O. Part B will use national data sets representative of the US population to determine environmental variables associated with childhood obesity. Part C will develop an instrument to measure family interrelationships that can be used clinically to predict weight loss and to compare patterns in families with and without obese children. The data from study 3 are essential to studies of the natural history, morbidity, and causes of childhood obesity.